When an inverter drives a rotating machine like a motor by using an inverter; a situation may be encountered such that the number of rotations of the rotating machine is considerably lowered by physical problems, for example, the rotating machine may be overloaded, or adhesion of lubricating oil in the rotating machine itself. A motor lock is known as a notable example of such conditions.
Patent Documents 1 and 2 are cited herein as disclosing techniques for dealing with the motor lock. In the technique described in Patent Document 1, a constant-voltage supply to the motor is stopped for a given period when the current flowing in the motor exceeds a reference value. In the technique described in Patent Document 2, a lock state is detected when a same pole of the motor is continuously detected for a predetermined time period.
Also, Patent Document 3 is cited as a technique about drooping control in which the rotating speed is controlled in such a manner that the current command does not exceed an upper-limit current value.    Patent Document 1: Japanese Utility Model Application Laid-Open No. 6-70441 (1994)    Patent Document 2: Japanese Patent Application Laid-Open No. 2005-245075    Patent Document 3: Japanese Patent Application Laid-Open No. 2002-138966
An inverter for controlling a motor uses switching elements, such as IGBTs. Allowable heat tolerances of such switching elements are becoming smaller because of the trends toward miniaturization and cost reduction.
When the conventional techniques cited above are used under such conditions, there are problems as mentioned below for switching elements, and further in the protection of the inverter.
In making a judgment as to whether the current flowing in the motor exceeds a simple reference value, carefully protecting the switching elements involves lowering the reference value. When the rotating speed is extremely low, the reference value has to be lowered because current flows for an increased time period in the same switching elements and the same motor winding. However, for high-speed rotation, the motor cannot run unless the reference value is exceeded, resulting in the reference value interfering with the operating region of the motor.
Also, when the condition is judged as being locked when the same pole of the motor has been continuously detected for a predetermined time period, the same judgment is made at the start of operation, causing a malfunction.
The present invention has been made by considering the problems above, and an object of the present invention is to make it possible, without interfering with the operating region of a motor, to more strictly judge inverter operation (excess input current) that deviates from the operating region in a low rotating speed area, so as to carefully protect the inverter.